1. Field of the invention
The present invention relates to a steam generating and steam cycle plant having a forced circulation steam generator, which has at least one evaporator, a separator, a blowdown tank and a feed line, with a feed unit, extending to the evaporator, from which separator a steam outlet line leads away to a steam treatment station, which separator is connected to the evaporator by means of a supply line. It also relates to a method of operating such a plant during a cold start with adequate quality of the working medium of the steam/water cycle, a method of cleaning the steam/water cycle during a cold start with inadequate quality of the working medium of the steam/water cycle, a method of operation during a warm start with adequate quality of the working medium and a method of cleaning the steam/water cycle during a warm start, full-load operation or part-load operation with inadequate quality of the working medium.
2. Discussion of Background
Impurities in the steam/water cycle of steam generating and steam cycle plant, for example steam power stations, can lead to deposits and corrosion and, in consequence, to operational interference and damage. For this reason, the operating medium for such plant must be cleaned.
In plant with drum boilers, impurities are removed by blowing- down the boiler water. This is based on the following principle. Evaporation of the water takes place in the boiler drum. Non-volatile substances remain in the boiler water and are concentrated by this. Blowing down the boiler water then removes these substances in concentrated form and therefore efficiently from the cycle.
In plant with forced circulation boilers, no water can be blown down--the feed water enters the boiler and leaves it as steam. In consequence, all the non-volatile substances remain in the boiler. Thus, deposits of iron oxide in the evaporator part are not unusual either. In the case of forced circulation boilers, therefore, condensate cleaning plants are usually inserted in the cycle; the condensate is filtered and, if appropriate, also desalinated by means of ion exchangers in these plants before being returned to the steam generator.
On cold start and run-down of forced circulation boilers, during evaporation instabilities and during part-load operation, under which conditions wet steam reaches the separator, recirculation of the boiler water takes place by means of a pump. Recirculation therefore takes place during start-up and run-down, for which purpose the water level in the separator must reach a required value. With an increasing supply of heat to the boiler, the steam generation increases and the circulating quantity decreases correspondingly.
In forced circulation boilers, superheated steam reaches the separator from a specified heat output onwards, so that pure forced circulation operation can take place.